When the reaction is at equilibrium ∴ΔG = Zero now we need to get Δ G°, we can get it from this formula: ΔG° = - RT㏑K(eq) when we have R = 8.315 J/mol°K and T in Kelvin = 25+273 = 298 so we need to get the K(eq) from this formula: K(eq) = concentration of products / concentration of reactants = [ C] [ D] / [A] [B] = (0.8*0.8) / (0.2*0.2) = 16 by substitution in ΔG° formula: ∴ ΔG° = - 8.315 * 298 *㏑(16) = -6870 Jmol^-1 /1000 = - 6.87 KJ mol^-1